The invention is directed generally to the field of avionics displays and, more particularly, to a method and apparatus to scan convert radar video from a polar sweep format into a horizontal raster format to achieve compatibility with a standard TV monitor.
Modern aircraft, and in particular, military aircraft, include a variety of display devices which are installed in the cockpit in order to provide information to the crew in a way which eases the crew workload. Such systems may include airborne digital map systems which are used on board various aircrafts for purposes of displaying aeronautical charts and other data such as navigational information. In other cockpit environments, radar information may be similarly displayed. FIG. 1 shows an avionic's suite of the type used on a modern aircraft including a heads up display 12, a left multifunction display (MFD) 14, a right MFD 16, mission computer 20, data bus 22, display computer 18, radar set 26 with associated discrete inputs 24 and radar graphics generator 10. In such systems, the mission computer 20 controls the functions of the display computer, graphics generator and radar set. The radar set accepts discrete inputs and provides outputs including video and serial digital data into the radar graphics generator which, in turn, sends the radar video to the display computer 18 for further processing and eventual display to the pilot via one of the display units. The key function of the radar graphics generator 10 is to convert the radar video outputs on lines 27a and 27b into the proper format required by the display computer 18. In one such system, the radar video outputs have a raster pattern of 675 lines with an update rate of 30/60 and must be converted into a 525 30/60 format. The primary source of control data is received by the radar graphics generator from the mission computer over bus 22 which may be, for example, a 1553B multiplexing bus. The mission computer communicates with and controls all the subsystems on the bus 22. The radar graphics generator receives mode information, radar attributes, and symbology insertion commands in the form of display macros. There are two primary links between the radar set 26 and the radar graphics generator. These links are the analog video carried on line 27a and the serial digital data transmitted on line 27B. In one known system, the analog interface transmits the radar video over a single triax cable. The video arrives in an RS-343 format comprising 512 active lines of the 675 line total. The radar video is transmitted in either a sector scan mode, or a back-and-forth orthogonal scan (B-Scan) patch raster. The sector scan modes require conversion from a polar format to a normal orthogonal raster, while the B-Scan is already orthogonal and only needs to be changed to a normal retrace raster presentation. The sector scan modes and B-Scan modes both require rotation prior to input into the display computer. The radar transmits digital target data to the radar graphics generator over a redundant bus consisting of six twisted pair leads. The interface consists of two mode lines, one megahertz clock line and the actual data line. The digital data and clock lines are repeated such that the digital data transmissions are redundantly received. The digital data messages are then converted into symbology and inserted into the radar video.
Typically included in the radar graphics generator 10 is a video scan converter. Current methods and apparatus scan convert radar data while the data is incoming. The radar data is then stored into a full field memory with X,Y addressing. This requires multiple writes of each radial or arc with slightly differing coordinates to prevent the occurrence of pixel "holes" where pixel holes are voids caused by missed X,Y coordinates during scan conversion. Besides the potential "holes" which are especially prevalent during zoom modes, current methods have problems accurately zooming and compressing the radar data. Also, in known apparatus, during scan conversions near the origin, multiple radar pixels can intersect an individual pixel.
The invention provided has advantages over the prior art which are achieved by storing the radar data indexed by radius and angle. Thus, in the full field memory there are no cases of "holes" or multiple writes to a single pixel. During the readout phase a high speed hardware pipeline performs the necessary warping from polar to cartesian coordinates, rotation and scaling. The pipeline utilizes bi-linear interpolation to ensure optimum display fidelity. Thus, the invention solves the difficult problems of missing pixels at the edges of the radar area and also prevents the loss of data near the origin due to multiple writes of pixels by different radials or arcs. The invention provides more accuracy especially in the area near the origin. The warping, rotation and scaling function are performed in real time by a much simpler pipeline than those available in the prior art. The pipeline must solve an inverse tangent function and solve the square root of the sum of X*X and Y*Y. Known pipeline processor method which solve these equations require as many as 12 steps and deliver insufficient accuracy for radar conversion applications. The method and apparatus of the invention require only seven steps including three steps for accessing the data and performing bi-linear interpolation.